The present invention relates to a lead frame made of copper or copper alloy on a raw material, a method for manufacturing the same and semiconductor device employing the same lead frame.
Generally, in a semiconductor lead frame made of mainly copper base metal such as copper, copper alloy or the like as its raw material, its chip loading region and inner lead portion are plated with silver. After chip bonding, wire bonding and resin sealing are finished, its outer lead portion is plated with solder. After that, an unnecessary portion of the lead frame is cut off and then individual semiconductor devices are separated.
Japanese Examined Patent Publication No. S63-49382 has disclosed a lead frame for a semiconductor device in which the surface of any one or both of a chip loading portion and wire bonding is coated with palladium or palladium alloy.
According to such prior art technique in which a chip loading portion and inner lead portion of a lead frame for use are plated with silver, then chip bonding, wire bonding and sealing with resin for the lead frame are carried out and then an outer lead portion is plated with solder before an unnecessary portion of the lead frame is cut out, formation of plating needs to be carried out at completely different two stages, that is, production stage of the lead frame and substantial finishing stage of production of a semiconductor device. Thus, the production cost thereof increases, thereby leading to a large reason for blocking a reduction of production cost of the semiconductor device.
On the other hand, according to the technique disclosed in the aforementioned Japanese Examined Patent Publication No. S63-49382, on production stage of the lead frame, that is, on a stage after the lead frame is formed, the chip loading portion, wire bonding portion and outer lead portion of the lead frame are plated with palladium or palladium alloy. In this case, because palladium or palladium alloy has an excellent solderability, it is not necessary to perform plating procedure to improve the solderability before the unnecessary portion of the lead frame is cut after sealing with resin. Therefore, the number of manufacturing works can be decreased.
However, in this technique disclosed in the aforementioned Japanese Examined Patent Publication No. S63-49382, unless the palladium is plated with in a quite large thickness, an influence of the nickel film thereunder is so strong that an anchor effect necessary for loading a chip cannot be obtained.
That is, when plating the chip loading region and the like with palladium or palladium alloy, a surface of raw material made of copper or the like cannot be directly plated therewith. After a ground layer is formed with nickel, the surface thereof must be plated with palladium or palladium alloy. In this case, usually the nickel plating is formed so as to have an elaborate, smooth film. The reason for this is that the elaborate smooth nickel layer needs to be formed so as to prevent an occurrence of defect such as pin hole or the like, which leads to formation of a local cell between a top surface and that layer. If the local cell is formed, corrosion is induced in the raw material.
However, if the plating film of palladium or palladium alloy is thin and if its ground layer is elaborate and smooth, the characteristics of palladium or palladium alloy, that is, hydrogen absorption characteristic, brittleness and the like thereof cannot be demonstrated sufficiently, so that the anchor effect necessary for the chip loading portion cannot be obtained. However, if the thickness of palladium or palladium alloy is increased, raw material cost increases because palladium is a very expensive metal. As a result, the price of the lead frame also increases, which is a serious problem.
Further, it is known that because the palladium has a catalytic action for gas, various gases, particularly organic gases, produced on assembly process of the semiconductor device are absorbed by palladium or palladium alloy film, so that the solderability of palladium or palladium alloy film deteriorates. To solve this problem, there is an art for forming a gold plating layer on the surface of the palladium or palladium alloy film. If the art of plating a top surface of a lead frame with gold is applied to the art of the Japanese Examined Patent Publication No.S63-49382, because the gold is a precious metal having a very high chemical stability, adhesive property between the sealing resin and this layer drops.
If the gold film is formed on the surface of the palladium or palladium alloy film as mentioned above, there occurs a worse influence than the bad influence resulting from a local cell between the lead frame raw material and the palladium or palladium alloy film, because the potential difference between the gold and the lead frame raw material is larger than the potential difference between the palladium or palladium alloy film and the lead frame raw material, and hence reliability is lowered.